The present invention generally relates to cannulas and more particularly pertains to cannulas suited for applications requiring a high degree of maneuverability as well as the ability to accommodate high blood flow rates therethrough with minimal blood damage.
Cannulas must often be able to satisfy a number of competing requirements. In applications such as for example, the inflow cannula of an implantable intravascular heart pump, the inner diameter of the cannula must be as large as possible in order to accommodate the extremely high flow rates inherent in such application. On the other hand, the outer diameter of the cannula should be as small as possible in order to enable it to be maneuvered through the convolutions of the patient's vasculature, for example around the aortic arch through the aortic valve and into the heart. Moreover, a smaller outer diameter minimizes the size of the puncture that must be made in the vasculature in order to gain access thereto. Additionally, the cannula must be stiff enough to allow its distal end to be routed through vasculature by manipulation of its proximal end, yet flexible enough to conform to the vasculature and not injure the tissue it comes into contact with. Blind retrograde insertion into for example, the left ventricle through the aortic valve is especially problematic in that an advancing cannula has a natural tendency to enter the sinus region adjacent the valve leaflet and become jammed rather than retrogradely passing through the periodically opened valve. Additionally, because substantial flow velocities and possible suction pressures may be involved, the risk of shear or cavitation must be addressed while the presence of a foreign body within the blood flow poses a risk of thrombogenisis especially in flow stagnant areas.
Cannulas have been previously devised in an attempt to satisfy the requirements associated with heart pump applications but have fallen short of overcoming many of the difficulties involved. The importance of preventing the radial collapse of the cannula had been recognized and consequently a spiral spring had previously been incorporated in some cannulas. On the other hand, in order to render the distal end as soft as possible, the spiral spring was terminated somewhat short of the distal end and while this does soften the distal end, the section of cannula sans spring was then prone to kinking, and consequently flow obstruction. The flexible tip is also subject to being sucked against parts of the vasculature or ventricular apparatus and can subsequently collapse to block the flow of fluid therethrough. In an effort to simultaneously impart the necessary flexibility and rigidity to the cannula, the wall thickness of hereto known cannulas has been varied along its length. By significantly increasing wall thickness near the proximal end, the necessary forces can be transmitted without sacrificing the flexibility needed at the distal end. However, while this imparts the desired stiffness differentiation, it has the undesired side effect of either significantly increasing the cannula's maximum external diameter or decreasing its minimum internal diameter. Additionally, the entrance ports formed in heretofore known cannulas have failed to take into consideration the substantial blood flow velocities that may be forced therethrough and the injury that may be sustained by the blood due to the abrupt directional changes that are encountered.
A cannula configuration is needed that is sufficiently soft and sufficiently rounded at the distal end to prevent injury yet not prone to collapse or wall suction. Moreover, the cannula must be sufficiently resistant to deformation to ensure maneuverability without sacrifice to its flow capacity. Blood flow into and through the cannula must be managed so as to minimize damage to the blood while minimizing pressure losses therewithin. Heretofore known cannulas have failed to adequately address these requirements.